Theft of electrically-powered equipment such as computers and related equipment such as monitors and printers, consumer home electronics such as televisions and VCR's, and scientific laboratory equipment has become a major problem in homes, businesses, and universities. This has led to the development of a large variety of security systems for the prevention of theft of such equipment.
There exist varyingly-effective general security systems which may detect the unauthorized entry of a person into a given area where such equipment is stored. Many of these systems employ motion detectors or heat detectors. However, these systems do not detect the removal of a specific piece of equipment. Security systems for individual items of equipment has ranged from detection of the removal of electrical power, to internal motion sensors with alarms, to cables which when moved alert a central alarm system. Most of these individual security systems however make regular use of the equipment more difficult, since false alarms are frequent when such equipment is slightly moved even by the authorized user.
Of course, there are relatively unsophisticated solutions to keeping a specific piece of equipment from being stolen, such as simply chaining down the equipment, but these are not convenient in all situations (for example, most homeowners are unlikely to lock their televisions in chains). Furthermore, once removed from such mechanical locking systems, such equipment can be freely used by the thief.
Because of the great need, numerous more sophisticated systems have been developed which allow the detection of theft of electrical equipment. All have their disadvantages, however.
Many of these prior art systems generally detect the connection of the equipment to an electric receptacle. In particular, many such security devices and systems have focused on the detection of electrical signals from the main power supply to the electrical device. For example, U.S. Pat. Nos. 4,945,341, 5,059,948 and 5,525,965 show a variety of systems where changes in the electrical state of the device are monitored. These systems have significant disadvantages which limits their usefulness.
For example, many such systems cannot distinguish power loss caused by removal of a power cord from a receptacle from power loss caused by a power outage or power failure. Since such power failures, even on a momentary basis, occur often, such systems provide many false alarms which may be not only be inconvenient but may also cause a security company or the Police to be less diligent in their monitoring of such alarms. Frequent false alarms require that such systems be reset often. Moreover, such false alarms due to power failures may occur when the users are not near the equipment, causing batteries to drain, limiting their future effectiveness.
Another example is the device disclosed in U.S. Pat. No. 5,418,521, which issued on May 23, 1995 to R. Read. Read discloses an extension cord-like power cable intended to be used with power tools to detect and signal their removal from the cord. In one embodiment of the Read cable, the male end thereof is plugged into an electrical socket in a normal fashion. The cable has a switch integrated into one current slot of the female socket.
The cable also has an integrated alarm which sounds when the tool is removed from the female socket of the cable. The switch is open when the tool is plugged-in, and closes when the tool is removed, closing a circuit which activates the alarm. In another embodiment, the male end of the cable has a similar switch to detect removal of the plug from the receptacle.
This cable and others like it, though, have many limitations which render them unsatisfactory for use in a home or office to prevent theft of electrical equipment such as a computer.
First, the Read cable is clearly intended to be used only in situations where the tool is far removed from the receptacle (ie. where the cable is long) since simply unplugging the cable from the receptacle defeats the alarm, at least in its simpler embodiment where there is no switch at the male end of the cable. Moreover, in some embodiments of Read's cable it appears that cutting power to the female end of the cable (for example, by turning off the alarm control switch near the male end of the cable or by turning off the power at the receptacle and merely cutting the cable between the female socket and the alarm) will prevent the alarm from sounding. This renders the cable useless where a determined thief is willing to chance cutting a live electrical cord.
Moreover, the switches used in the Read cable, particularly the one used at the male end, are easily manipulated or defeated by inserting a thin card, for example, between the switch and the receptacle. Also, the alarm is transitory and sounds only when the switches are closed; a thief need only quickly replace the power tool plug with another plug in the female socket, for example, to shut the alarm off. This might take a second, at most.
Also, in one embodiment there is no way to deactivate the alarm without unplugging the cable from the electrical outlet, so even the authorized power tool user will set off the alarm by switching power tools, at least until the new tool is plugged in, unless the cable is first unplugged at the receptacle. Finally, although there is one embodiment of the Read invention which does have a switch to deactivate the system, this switch is easy for a thief to find, rendering the system useless.
There are other very complicated security systems which have been suggested. For example, U.S. Pat. No. 4,680,574 discloses an appliance anti-theft system which uses time domain reflectrometry to determine when the length of the power cord has been altered, for example, by cutting. However, such systems are very complicated and expensive.
There remains, accordingly, a need for a theft detection system conveniently forming a power cord for an electrical device which has the following elements:
1. the ability to detect the removal of the cord from an electric receptacle, and also possibly the removal of at least a portion of the cord from the electrical device (such a cord may have a female end which the electrical device is plugged into, or may be itself built into the electrical device); PA0 2. an integral alarm which is activated by removal of the device from the cord, or the removal of the cord from the receptacle; PA0 3. a system for deactivating the alarm to allow authorized removal of the cord from the receptacle and the device from the cord; PA0 4. the ability to distinguish between a lack of electrical conduction in the cord caused by removal from the receptacle, and that caused by the lack of source electricity (for example, caused by a power outage); PA0 5. the ability to detect the cutting of the cord; PA0 6. a mechanical configuration which does not allow manipulation of the sensing switches of the device; and PA0 7. a low production cost;